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Programming with Collections Collections in Java Using Arrays Week 9

Programming with Collections Collections in Java Using Arrays Week 9. Programming with Collections. CONCEPTS COVERED THIS WEEK. Arrays Using the For Loop with Arrays. Programming with Collections. FIXED-SIZE COLLECTIONS. Sometimes the maximum collection size can be pre-determined.

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Programming with Collections Collections in Java Using Arrays Week 9

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  1. Programming with Collections Collections in Java Using Arrays Week 9

  2. Programming with Collections CONCEPTS COVERED THIS WEEK • Arrays • Using the For Loop with Arrays

  3. Programming with Collections FIXED-SIZE COLLECTIONS • Sometimes the maximum collection size can be pre-determined. • Programming languages usually offer a special fixed-size collection type: an array. • Java arrays can store either objects or primitive-type values (e.g. int, float, etc…) • Arrays use a special syntax.

  4. What is an array? • A special type of data structure which consists of a group of items of the same type, e.g. int, double, String etc. • We can manipulate the whole group as a single entity and we can use any single member of that group (called an array element) independently. • Each element is located by an INDEX or SUBSCRIPT pointing to the appropriate ‘cell’ number.

  5. Using Arrays • Declaring & initialising an array called numbers that has 10 integer elements: int[ ] numbers; numbers = new int [10]; • This tells the system to create 10 linked integer memory locations that can be accessed by the name numbers[..]

  6. The numbers array • numbers 0 0 0 0 0 0 0 0 0 0 0 1 2 3 4 5 6 7 8 9 The statement: numbers[5] = 8; puts 8 in element 5 0 0 0 0 0 8 0 0 0 0 0 1 2 3 4 5 6 7 8 9

  7. Note • The subscripts (elements) are numbered from 0 to 9, so subscript number 5 is the sixth one along. • Array subscripts ALWAYS start at 0. • Arrays have square brackets [ ]. • Uninitialised numeric array elements take the value 0 by default.

  8. Creating an array • Choose any legal variable name • Key in [ ] square brackets before (left) or after (right) of the name. • int ages[ ]; • String[ ] names; • ages = new int [6]; • names = new String [5]; • Numbers in [ ] are indices - they tell how many items BUT can also be which element is referred to

  9. Using Arrays • Square-bracket notation is used to access an array element: c[5] • Elements are used like ordinary variables. • On the left of an assignment: • c[5] = 8; • In an expression: • int adjusted = c[5] – 3; • c[5]++;

  10. Another example int c[]; c = new int[12]; //to ‘load’ each element we could use 12 separate statements c[0] = -45; c[1] = 6; c[2] = 0; c[3] = 72; c[4] = 1543; c[5] = -89; c[6] = 0; c[7] = 62; c[8] = -3; c[9] = 1; c[10] = 6453; c[11] = 78; • Alternatively, we could initialise (‘populate’) the array when we declare it: int c[] = {-45, 6, 0, 72, 1543, -89, 0, 62, -3, 1, 6453, 78};

  11. Arrays and for loops • Suppose we wanted to display all the elements of array c[], we could write 12 System.out.print statements OR use a ‘for’ loop: for (int i = 0; i < c.length; i++) System.out.print ( c[i] + “ ” ); • c.length returns the number of elements in the array, 12 in this case

  12. Arrays and for loops • To add all the elements of array c[] and put the answer in sum: int sum = 0; for (int i = 0; i < c.length; i++) sum += c[i]; System.out.println (“The sum is ” + sum);

  13. Arrays & for loops //To find the largest element in c int largestValue = c[0]; int indexOfLargest = 0; for (int i = 0; i < c.length; i++) { if (c[i] > largestValue) { largestValue = c[i]; indexOfLargest = i; } }

  14. Good Programming Tip • It’s better to declare the size of the array independently of the array declaration, e.g. int ARRAY_SIZE = 12; int c[]; c = new int [ARRAY_SIZE]; • Then, to change the size, you only need to change one line.

  15. Arrays Summary • Arrays are appropriate where a fixed-size collection of data items is required. • Arrays use special syntax. • The subscript always starts at 0. • For loops are often used to iterate over arrays.

  16. Common Array Tasks • Create • Display contents • Search • Sum (add all numeric elements) • Sort

  17. Programming with Collections REVIEW OF ARRAYS • Arrays are appropriate where a fixed-size collection is required. • Arrays use special syntax. • For loops offer an alternative to while loops when the number of repetitions is known. • For loops are often used to iterate over arrays.

  18. Programming with Collections ARRAYS vs ARRAYLISTS • An array has to be given its size at the time it is created. An array list does not have a pre-determined size. • To place an item in an array you need to place it in a specific element location. With an array list you just use the add method. • An array can store either objects or primitive data types but an array list can only store objects.

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